The subject invention is directed generally to holographic exposure systems, and more particularly to a constant wavelength holographic exposure system for producing constant wavelength holograms on spherically curved recording media.
Holographic optical elements including constant wavelength spherically curved holograms (i.e., holograms supported by a spherically curved substrate) are utilized for various applications including military head-up displays, visors, and device and eye protection filters.
Curved constant wavelength holograms are presently made pursuant to flood exposure systems wherein the entire holographic recording medium is subjected to exposure illumination at the same time. In order to obtain constant wavelength over the entire format of a hologram generated on a spherically curved substrate, the exposure must be made with a point source located coincident with the center of curvature of the recording film, or with very complex optics that provide for a constant angle of incidence of the exposure illumination at all points of the recording film.
In view of the specific requirements for exposure of a spherically curved constant wavelength hologram, such a hologram would be very difficult to make pursuant to a known technique commonly called "angle tuning" wherein exposure is made with an available wavelength of light and caused to impinge the hologram recording film at an angle from the film normal which subsequently produces fringe spacings greater than would be obtained had the same wavelength exposure light impinged the film at a normal angle of incidence. This produces a hologram at a higher wavelength than would be otherwise possible for the available exposure wavelength (neglecting any film thickness changes resulting from other phases of the hologram production process which follow the exposure).
An "angle tuned" spherically curved constant wavelength might possibly be made by scanning the format with an unexpended exposure beam from a laser that can be wavelength tuned during the exposure process to compensate for fringe spacing changes resulting from variations in incident angle that the exposure beam would experience during the scan. However, such exposure arrangement would be exceedingly difficult to realize.